Waste Management Strategy - Comhairle nan Eilean Siar

What is composting
Composting is the biological process in which micrco-organisms convert degradable organic matter into carbon dioxide and
water vapour using oxygen in the air and leaving a stabilised residue known as compost.
And in-vessel composting
With in-vessel composting the process happens inside a sealed container and computerised controls optimise levels of
moisture, air temperature and mixing. Emissions are controlled and the time required for composting is less than with
traditional pit style systems. The end product is also usually of a higher quality and is therefore more useful.
5.6 Option 4: Centralised Anaerobic Digestion
This option is an extension to Option 3. An anaerobic digestion plant in Stornoway would use the organic fraction of the waste
to produce methane that would then generate electricity. As with option 3 all areas would either have a weekly collection by a
split RCV or access to a bring site. Recyclable materials such as glass, paper, plastics and metals will once again be collected
separately with all other non-biodegradable wastes being transported for landfilling at Bennadrove.
If some form of pasteurisation system could be added to the plant aquaculture wastes could also be processed.
BPEO Considerations
• Environmental All emissions are controlled. Energy recovery. Potential for long-term land improvement scheme. Some
additional transportation required.
• Economic Split RCVs would need to be purchased. Additional income from accepting aquaculture waste from throughout
Scotland and from accepting some wastes from western mainland. Potential to sell landfill permits. Saving in energy costs
due to electricity production.
• Social Producers held responsible for their waste. New skills required. Recycling improved in some islands. Definite
potential for employment in plant and shipping.
• Practicability Technology not proven for municipal waste in the UK although well proven on the Continent. Flexible to
changing circumstances.
• Compliance with other Policies All biodegradable municipal waste disposed of in this manner, therefore all landfill
targets met.
What is anaerobic digestion
Anaerobic digestion is a biological process in which micro-organisms convert degraded organic matter into carbon dioxide,
methane, water, heat in the absence of oxygen, and leaves a residue known as digestive which can be used as a soil
conditioner. Liquid fertiliser is also produced. The methane is collected and used as a fuel and there is the potential for local
heat use.
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Western Isles Waste Options Paper